Superabsorbent polymers (SAPs) are well known in the art. They are commonly applied in absorbent articles, such as diapers, training pants, adult incontinence products and feminine care products to increase the absorbent capacity of such products while reducing their overall bulk. The SAPs generally are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight.
Commercial production of SAPs began in Japan in 1978. The early superabsorbent was a cross-linked starch-g-polyacrylate. Partially neutralized polyacrylic acid eventually replaced earlier superabsorbents in the commercial production of SAPs, and is the primary polymer employed for SAPs today. SAPs are often applied in form of small particles, such as fibers or granules. They generally consist of a partially neutralized lightly crosslinked polymer network, which is hydrophilic and permits swelling of the network once submerged in water or an aqueous solution such as physiological saline. The cross-links between the polymer chains assure that the SAP does not dissolve in water.
It is well known in the art to coat the surface of the SAP particle in order to add further functionality: Silica or aluminum sulphate can be applied as well as waxes, polymers or inorganic water-insoluble salts. The coating can be done in a separate process step after the surface cross-linking, or the coating substance can be applied together with the surface cross-linking solutions.
In the case of aluminum sulphate, ionic bonds with the carboxylate ions of the SAP particle are formed, but those bonds are ruptured in the presence of body fluids due to solvation effects. In the case of most waxes and polymers, predominantly weak and reversible van-der-Waals forces provide the attachment to the surface.
In all cases, however, the coatings are merely reversibly attached to the surface of the SAP particles and no irreversible covalent bonds are created between the coating and the SAP particles. Therefore, such coatings tend to be relatively unstable: They may be washed off easily upon contact with liquid or the coating may be eliminated partly or completely during conveying of the SAP particles (e.g. during manufacture or transportation of an absorbent article comprising the SAP particles) by a mechanical means such as abrasion.
If the SAP particles are applied in absorbent articles, the surface properties of the SAP particles are negatively altered after wash off of the coating upon contact with body fluids during use of the absorbent article. Furthermore, the overall performance of the absorbent article may be negatively affected by the coatings dissolved in the body fluids
For example, the surface tension of the body fluids may be reduced due to the presence of the washed off coating. This, in turn, may decrease the capillary pressure of the body fluids and as a result, the porous absorbing and/or desorbing materials in the absorbent article may partly or completely lose their wicking properties.
Or, a colored pigment comprised by the coating, which gets dissolved in the body fluids, may negatively affect the aesthetic appearance of the absorbent article or the color may irritate the wearer/caretaker.
Especially, if the coating is done with waxes, the surface properties of the SAP particles, such as permeability for liquids, may be negatively affected, depending e.g. on the thickness of the wax coating.
Another drawback with coatings is that they may diffuse inside the SAP particle during the coating process, depending e.g. on the duration of the coating process.
Also known in the art are absorbent gelling particles comprising a water-insoluble, absorbent, hydrogel-forming polymer and a polycationic polymer, wherein the polycationic polymer is covalently bound to the absorbent gelling particles. The covalent bonds are created via carboxyl groups comprised especially at the surface of said absorbent gelling particles. However, often a relatively large portion of the carboxyl groups of the absorbent gelling material is neutralized prior to the polymerization step, and hence, commonly relatively few carboxyl groups are available for the covalent bond between the absorbent gelling particles and the polycationic polymer. Consequently, the degree of neutralization has to be balanced with the need to covalently bond the polycationic polymer to the absorbent gelling particles, because both process steps make use of the carboxyl groups.
To overcome the problems set out above, it is an objective of the present invention to provide SAP particles with added or modified functionalities, wherein the functionalities are added or modified permanently. The functionality shall not get lost due to wash off upon contact of the SAP particles with liquid. Moreover, the functionality shall also be provided durably in that it does not get lost upon conveying and transport of the SAP particles and during manufacture, e.g. of absorbent articles comprising such SAP particles.
Furthermore, the functionality shall be provided without the need to compromise on the degree of neutralization of the SAP particles.
It is a further objective of the present invention, to provide a method to produce such SAP particles with added or modified functionality.